Generally, if you had an allowable capital loss in a year, you have to apply it against your taxable capital gain for that year. If you still have a loss, it becomes part of the computation of your net capital loss for the year. You can use a net capital loss to reduce your taxable capital gain in any of the 3 preceding years or in any future year.

Unused 2018 net capital losses can be carried back to 2015, 2016, and 2017 without adjustment, but if unused net capital losses of other years are carried forward and applied to your 2018 taxable capital gains, you have to determine your adjustment factor, because the inclusion rate may have changed.